Electroplating of a metal, particularly copper electroplating, requires an electrically conductive seed layer in case of substrate surfaces made of typical barrier materials such as ruthenium and cobalt. Such a seed layer can in principle consist of any kind of electrically conductive material. However, copper seed layers are preferred because of the high intrinsic electrical conductivity of copper.
Said seed layer has to fulfil several requirements such as a sufficient adhesion to the underlying barrier layer and to the metal deposited on top of said seed layer by electroplating. Furthermore, the seed layer should have a homogeneous (narrow) thickness distribution and a smooth outer surface. Such requirements are of particular importance in the manufacture of microchips and the like, where recessed structures which need to be coated with such a copper seed layer can have dimensions as low as in the nanometer range.
Methods for plating a copper seed layer onto a barrier layer made of ruthenium are disclosed in U.S. Pat. No. 7,998,859 B2 and U.S. Pat. No. 7,470,617 B2. Both methods utilize an aqueous solution comprising a reducing agent to remove undesired surface oxides from the ruthenium barrier layer prior to electroless copper plating. Both methods further utilize standard electroless copper plating bath composition comprising either NaOH of KOH as the sole source for hydroxide ions. The roughness of the outer surface of the copper seed layer in both cases is too high (comparative Examples 1 to 4).
Accordingly, there is a need to provide a thin copper layer deposited by electroless plating onto a barrier layer which results in a copper seed layer having an outer surface with an improved smoothness.